The field of the present invention is surgical cannulas used to support the flow of blood, as in a hemodialysis. In particular, the present invention relates to surgical cannulas adapted simultaneously to withdraw and return blood through a single cannula, thereby permitting a single venipuncture.
The duplex needle of U.S. Pat. No. 2,137,132 has a dual tubular construction, but the two tubes merge within the needle. Blood cannot be withdrawn through one tube and returned through the second tube of the same needle. Instead, the treatment of blood in U.S. Pat. No. 2,137,132 requires two separate needles. The fluid inlet-outlet device of Curtis U.S. Pat. No. 2,409,343 also does not disclose a suitable method for withdrawing and returning blood to a body. Instead, it is designed "for piercing self-sealing insertion through a puncturable closure of a container," and does not disclose means for overcoming the problems associated with the withdrawal of blood from a blood vessel. In the case of the Curtis device the rigid walls of the container will not collapse during withdrawal of the fluid, and there is no danger of blockage of the intake opening. On the other hand, in the case of a cannula inserted into a blood vessel, the walls of the blood vessel collapse and tend to block the intake opening. The Curtis patent teaches that an intake opening be as open as possible, with preferably nothing to interfere with the withdrawal of fluid. The Curtis device would not, therefore, be an acceptable means for the withdrawal of blood from a blood vessel due to the collapsible nature of the blood vessel walls. Additionally, the Curtis device is not suitable for insertion into a human or other animal body due to the danger of multiple punctures resulting from exposed tips of the two side-by-side tubes.
The prior art reveals several methods by which blood can be withdrawn and returned to the body. First, the blood can be withdrawn from an artery and returned to the vein by means of two separate surgical implantations. This external by-pass is called a shunt. The second method of blood withdrawal involves a fistula. The fistula method entails an internal surgical connection between an artery and a vein. Because of the higher blood pressure in an artery, the vein will expand below the point of connection. This area of expansion has the appearance of a blood bubble, and will be referred to as the fistula. The fistula method calls for an intake needle to be inserted into the fistula against the flow of blood. In dual venipuncture devices a second needle (the outflow needle) is then inserted into a different vein downstream, and placed in the same direction as the flow of blood. In the second of the above methods, two separate needles are used, requiring two separate punctures.
The prior art discloses various methods for single needle dialysis. Examples may be found in U.S. Pat. Nos. 3,756,234 and 3,830,234, which discuss in general terms only a hollow cannula where the separation of arterial and venous branches is done via a T-joint and valves. These valves alternately open and close with pressure build-up, resulting in time separated pulses between the arterial and venous branches.